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+# `<nixpkgs/nixos/lib/make-disk-image.nix>` {#sec-make-disk-image}
+
+`<nixpkgs/nixos/lib/make-disk-image.nix>` is a function to create _disk images_ in multiple formats: raw, QCOW2 (QEMU), QCOW2-Compressed (compressed version), VDI (VirtualBox), VPC (VirtualPC).
+
+This function can create images in two ways:
+
+- using `cptofs` without any virtual machine to create a Nix store disk image,
+- using a virtual machine to create a full NixOS installation.
+
+When testing early-boot or lifecycle parts of NixOS such as a bootloader or multiple generations, it is necessary to opt for a full NixOS system installation.
+Whereas for many web servers, applications, it is possible to work with a Nix store only disk image and is faster to build.
+
+NixOS tests also use this function when preparing the VM. The `cptofs` method is used when `virtualisation.useBootLoader` is false (the default). Otherwise the second method is used.
+
+## Features {#sec-make-disk-image-features}
+
+For reference, read the function signature source code for documentation on arguments: <https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/make-disk-image.nix>.
+Features are separated in various sections depending on if you opt for a Nix-store only image or a full NixOS image.
+
+### Common {#sec-make-disk-image-features-common}
+
+- arbitrary NixOS configuration
+- automatic or bound disk size: `diskSize` parameter, `additionalSpace` can be set when `diskSize` is `auto` to add a constant of disk space
+- multiple partition table layouts: EFI, legacy, legacy + GPT, hybrid, none through `partitionTableType` parameter
+- OVMF or EFI firmwares and variables templates can be customized
+- root filesystem `fsType` can be customized to whatever `mkfs.${fsType}` exist during operations
+- root filesystem label can be customized, defaults to `nix-store` if it's a Nix store image, otherwise `nixpkgs/nixos`
+- arbitrary code can be executed after disk image was produced with `postVM`
+- the current nixpkgs can be realized as a channel in the disk image, which will change the hash of the image when the sources are updated
+- additional store paths can be provided through `additionalPaths`
+
+### Full NixOS image {#sec-make-disk-image-features-full-image}
+
+- arbitrary contents with permissions can be placed in the target filesystem using `contents`
+- a `/etc/nixpkgs/nixos/configuration.nix` can be provided through `configFile`
+- bootloaders are supported
+- EFI variables can be mutated during image production and the result is exposed in `$out`
+- boot partition size when partition table is `efi` or `hybrid`
+
+### On bit-to-bit reproducibility {#sec-make-disk-image-features-reproducibility}
+
+Images are **NOT** deterministic, please do not hesitate to try to fix this, source of determinisms are (not exhaustive) :
+
+- bootloader installation have timestamps
+- SQLite Nix store database contain registration times
+- `/etc/shadow` is in a non-deterministic order
+
+A `deterministic` flag is available for best efforts determinism.
+
+## Usage {#sec-make-disk-image-usage}
+
+To produce a Nix-store only image:
+```nix
+let
+  pkgs = import <nixpkgs> {};
+  lib = pkgs.lib;
+  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
+in
+  make-disk-image {
+    inherit pkgs lib;
+    config = {};
+    additionalPaths = [ ];
+    format = "qcow2";
+    onlyNixStore = true;
+    partitionTableType = "none";
+    installBootLoader = false;
+    touchEFIVars = false;
+    diskSize = "auto";
+    additionalSpace = "0M"; # Defaults to 512M.
+    copyChannel = false;
+  }
+```
+
+Some arguments can be left out, they are shown explicitly for the sake of the example.
+
+Building this derivation will provide a QCOW2 disk image containing only the Nix store and its registration information.
+
+To produce a NixOS installation image disk with UEFI and bootloader installed:
+```nix
+let
+  pkgs = import <nixpkgs> {};
+  lib = pkgs.lib;
+  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
+  evalConfig = import <nixpkgs/nixos/lib/eval-config.nix>;
+in
+  make-disk-image {
+    inherit pkgs lib;
+    config = evalConfig {
+      modules = [
+        {
+          fileSystems."/" = { device = "/dev/vda"; fsType = "ext4"; autoFormat = true; };
+          boot.grub.device = "/dev/vda";
+        }
+      ];
+    };
+    format = "qcow2";
+    onlyNixStore = false;
+    partitionTableType = "legacy+gpt";
+    installBootLoader = true;
+    touchEFIVars = true;
+    diskSize = "auto";
+    additionalSpace = "0M"; # Defaults to 512M.
+    copyChannel = false;
+    memSize = 2048; # Qemu VM memory size in megabytes. Defaults to 1024M.
+  }
+```
+
+